The procuticle is much thicker than the epicuticle, and is generally composed of protein and α‑
chitin covalently bonded to each other to form complex glycoprotein. This major layer of the
exoskeleton is the main contributor to the overall strength and rigidity, and can be further divided
into two sublayers; the exocuticle and the endocuticle. The exocuticle is directly beneath the
procuticle and may be hardened in areas by sclerotisation. The endocuticle differs in that it is
thicker, has less protein, more chitin, and is often strengthened by mineralisation.